feat: integrate Stripe payment with bugfixes and active timeout cancellation

- Add Stripe payment provider with Checkout Session flow
- Payment provider abstraction layer (EasyPay + Stripe unified interface)
- Stripe webhook with proper raw body handling and signature verification
- Frontend: Stripe button with URL validation, anti-duplicate click, noopener
- Active timeout cancellation: query platform before expiring, recover paid orders
- Singleton Stripe client, idempotency keys, Math.round for amounts
- Handle async_payment events, return null for unknown webhook events
- Set Checkout Session expires_at aligned with order timeout
- Add cancelPayment to provider interface (Stripe: sessions.expire, EasyPay: no-op)
- Enable stripe in frontend payment type list

package.json

@@ -7,6 +7,9 @@
     "build": "next build",
     "start": "next start",
     "lint": "eslint",
+    "typecheck": "tsc --noEmit",
+    "format": "prettier --write .",
+    "format:check": "prettier --check .",
     "test": "vitest run",
     "test:watch": "vitest"
   },
@@ -18,6 +21,7 @@
     "qrcode": "^1.5.4",
     "react": "19.2.3",
     "react-dom": "19.2.3",
+    "stripe": "^20.4.0",
     "zod": "^4.3.6"
   },
   "pnpm": {
@@ -37,6 +41,7 @@
     "@vitejs/plugin-react": "^5.1.4",
     "eslint": "^9",
     "eslint-config-next": "16.1.6",
+    "prettier": "^3.8.1",
     "prisma": "7.4.1",
     "tailwindcss": "^4",
     "typescript": "^5",